This invention relates to apparatus for and a method of treating wellbores, particularly treatments to reduce or prevent wellbore instability.
Various caliper tools for gauging the diameter of a borehole are known in the art. In one example, a caliper tool includes one or more bow springs coupled to a tool body. When the tool body is disposed in a borehole, the bow spring engages the borehole wall and expands and contracts as the tool body traverses the borehole and the borehole diameter changes. The motion of the bow spring can provide an indication of the borehole diameter. In this case, a sensing device can be attached to the bow spring and used to monitor the motion of the bow spring. This is taught, for example, in U.S. Pat. Nos. 2,639,512 and 7,069,775 B2. Some caliper tools further include one or more rigid arms coupled between the tool body and the bow spring. The rigid arm deflects as the bow spring expands and contracts, and the motion of the rigid arm provides an indication of the borehole diameter.
Wellbore instability takes many forms, can have several causes and may be treated in many ways. Uneven earth stresses and specific rock properties can combine to cause breakouts in a wellbore (i.e. the cross-section of the wellbore is enlarged along one axis while the rest of the borehole remains in-gauge or near to gauge). In drilling the overburden, this phenomenon can lead to major well construction problems including stuck pipe and poor zonal isolation. In a reservoir, the same problems can occur as in the overburden, but the additional inconvenience of sand production can also take place, sometimes throughout the life of the well.
Known wellbore treatment methods can be mechanical, e.g, setting a sacrificial casing string or expandable tubular, sand screen etc., chemical, e.g. injection of resins to consolidate sand, changes in the drilling fluid chemistry to prevent further instability etc., or Theological, e.g. by placing gels into the wellbore to stabilize fractured rock and similar methods.
These treatments meet with varying degrees of success, but are generally applied to the whole wellbore within a defined depth range. Preserving formation permeability can be important (for instance in a producing reservoir), but this is more often than not severely compromised by current treatments, or an additional step of perforating past the treatment is required to re-establish the wellbore-reservoir connection.